1. Field of the Invention
The present invention relates to an apparatus for determining a deterioration of a three-way catalyst arranged in an exhaust passage of an internal combustion engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 61-286550 discloses an apparatus for determining a deterioration of a three-way catalyst, wherein the three-way catalyst converter for purifying exhaust gas is arranged in an exhaust passage of an internal combustion engine, an upstream O.sub.2 sensor is arranged in the exhaust passage upstream of the three-way catalyst converter, and a downstream O.sub.2 sensor is arranged in the exhaust passage downstream of the three-way catalyst converter. The upstream and downstream O.sub.2 sensors detect a concentration of oxygen in the composition of the exhaust gas. In this apparatus, it is determined that the condition of the catalyst has deteriorated when a ratio of a cycle of a reversion of an output of the upstream O.sub.2 sensor to a cycle of a reversion of an output of the downstream O.sub.2 sensor is larger than a predetermined value.
In this apparatus, however, a problem arises in that it cannot determine a considerable deterioration of the condition of the catalyst, i.e., whether or not the purification rate of HC has become smaller than 40-50%, for the following reasons.
In the usual air-fuel ratio feedback control, an air-fuel ratio feedback correction amount is calculated to obtain a proper control cycle of the air-fuel ratio, and the air-fuel ratio is controlled by the air-fuel ratio feedback correction amount. The rate of change of the air-fuel ratio feedback correction amount, i.e., the change of the air-fuel ratio feedback correction amount per unit time, is relatively large and thus the rate of change of the air-fuel ratio, i.e., the change of the air-fuel ratio per unit time, is also relatively large.
In this air-fuel ratio feedback control, since a full O.sub.2 storage amount is reduced when the condition of the catalyst has deteriorated, the oxygen stored in the catalyst is consumed in a relatively short time. Accordingly, since the cycle of the reversion of the output of the downstream O.sub.2 sensor is shorter than a predetermined time, it can be determined that the condition of the catalyst has deteriorated.
Nevertheless, since the rate of the change of the air-fuel ratio is relatively large, the oxygen stored in the catalyst is consumed in a short time when the condition of the catalyst has considerably deteriorated, i.e., when the purification rate of HC is smaller than 40-50%, similar to the state when the condition of the catalyst has slightly deteriorated, i.e., when the purification rate of HC is smaller than 80%. Therefore, the cycle of a reversion of the output of the downstream O.sub.2 sensor when the condition of the catalyst has considerably deteriorated is substantially similar to the cycle of a reversion of the output of the downstream O.sub.2 sensor when the condition of the catalyst has slightly deteriorated.
As a result, this apparatus cannot distinguish between a slight deterioration of the condition of the catalyst and a considerable deterioration of the condition of the catalyst.